1. Field of the Invention
The present invention relates generally to a process for the hydrodesulfurization of a diesel boiling range stream in a distillation column reactor. More particularly the invention relates to a process wherein a diesel boiling range fraction is fed to a distillation column reactor containing a hydrodesulfurization catalyst where the organic sulfur compounds contained in the diesel fraction are reacted with hydrogen to form H2S which can be stripped from the overhead product.
2. Related Information
Petroleum distillate streams contain a variety of organic chemical components. Generally the streams are defined by their boiling ranges which determine the compositions. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated materials (diolefins). Additionally, these components may be any of the various isomers of the compounds.
Organic sulfur compounds present in these petroleum fractions are denoted as xe2x80x9csulfurxe2x80x9d. The amount of sulfur is generally dependent on the crude source. For instance the Saudi Arabian crudes are generally high in sulfur as are certain domestic crudes. Kuwaiti, Libyan and Louisiana crudes are generally low in sulfur. The type of sulfur compound will also depend on the boiling range of a given stream. Generally the lower boiling range fractions contain mercaptans while the higher boiling fractions contain thiophenic and heterocyclic sulfur compounds.
A diesel range fraction is defined by a boiling range of from about 450-650xc2x0 F. Because the cracked diesel boiling range material from a cracked stream contains a high degree of unsaturates and cyclic compounds it is not suitable for diesel fuel without further treatment. For this reason the cracked xe2x80x9cdieselxe2x80x9d is sold as heating oil.
The organic sulfur compounds are almost always considered to be contaminants. They hinder downstream processing and at the very least make noxious SO2 gas when burned. The degree of removal is dependent upon the use of the fraction. In the case of diesel or heating oil the desire is to prevent SO2 upon combustion. For this reason the current EPA regulations call for combustible motor fuel such as gasoline, kerosene or diesel to have not more than about 500 wppm sulfur. The same limit is placed upon heating oil.
The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. In the past this has generally been done by downflow over fixed beds concurrently with copious quantities of hydrogen in the feed. The following reactions illustrate the typical reactions in a prior art HDS unit:
RSH+H2xe2x86x92RH+H2Sxe2x80x83xe2x80x83(1)
RCl+H2xe2x86x92RH+HClxe2x80x83xe2x80x83(2)
2RN+4H2xe2x86x92RH+NH3xe2x80x83xe2x80x83(3)
ROOH+2H2xe2x86x92RH+2H2Oxe2x80x83xe2x80x83(4)
Additional reactions depend upon the sulfur compounds present and the source of the fraction. For example the desulfurization of thiophenes and other heterocyclic sulfur compounds necessarily involves breaking and saturation of the rings. Typical operating conditions for the standard fixed downflow reactors are:
After the hydrodesulfurization is complete the product is fractionated or simply flashed to release the hydrogen sulfide and collect the now sweetened fraction. The hydrogen sulfide can be converted to elemental sulfur by conventional means.
The use of a distillation column reactor to remove sulfur from a diesel boiling range stream is disclosed in commonly owned U.S. Pat. No. 5,779,883 (see example 3) where the catalyst was placed into the middle section of a distillation column reactor and the liquid feed was to the middle of the bed or below the bed. The sulfur conversion rate was 78%.
The invention is an improvement to the process disclosed in U.S. Pat. No. 5,779,883 wherein the liquid diesel to the distillation column reactor is fed above the catalyst bed and the hydrogen fed below. In addition in this mode there is no need to add heat via a reboiler as the exothermic heat of reaction of hydrodesulfurization provides the energy necessary for the distillation. Briefly the invention is a process for the hydrodesulfurization of a diesel boiling range petroleum fraction which comprises:
(a) feeding a diesel boiling range petroleum fraction to a distillation column reactor containing a bed of hydrodesulfurization catalyst at a point above said bed;
(b) feeding hydrogen to said distillation column reactor at a point below said bed;
(c) concurrently in said distillation column reactor
(1) distilling said diesel boiling range petroleum fraction whereby there are vaporous petroleum products rising upward through said distillation column reactor, an internal reflux of liquid flowing downward in said distillation column reactor and condensing products within said distillation column reactor, and
(2) contacting said diesel boiling range petroleum fraction and said hydrogen in the presence of a hydrodesulfurization catalyst at a total pressure of less than about 300 psig, hydrogen partial pressure in the range of 0.1 to less than 80 psi and a temperature in the range of 400xc2x0 to 800xc2x0 F. whereby a portion of the organic sulfur compounds contained in said diesel boiling range petroleum fraction react with hydrogen to form H2S;
(d) withdrawing an overheads from said distillation column reactor containing said H2S;
(e) separating the H2S from said overheads by condensing a higher boiling fraction of said overheads;
(f) returning a portion of said condensed higher boiling fraction of said overheads to said distillation column reactor as reflux; and
(g) withdrawing a bottoms product having a lower sulfur content that said diesel boiling range petroleum fraction.
In one embodiment the heat necessary for distilling said diesel boiling range petroleum fraction is provided by the heat of reaction of said organic sulfur compounds with said hydrogen.
For the purposes of the present invention, the term xe2x80x9ccatalytic distillationxe2x80x9d includes reactive distillation and any other process of concurrent reaction and fractional distillation in a column regardless of the designation applied thereto. Several different arrangements have been disclosed to achieve the desired result. For example British Patents 2,096,603 and 2,096,604 disclose placing the catalyst on conventional trays within a distillation column. A series of U.S. patents, including particularly U.S. Pat. Nos. 4,443,559 and 4,215,011, exemplify using the catalyst as part of the packing in a packed distillation column.
The catalyst beds as used in the present invention may be described as fixed, meaning positioned in fixed area of the column and include expanded beds and ebulating beds of catalyst. The catalysts in the beds may all be the same or different so long as they carry out the function of hydrogenation as described. Catalysts prepared as distillation structures are particularly useful in the present invention.
If desired or required the H2S may be stripped from the overhead product in a separate distillation column.